This invention relates to tunable metamaterials, nonlinear metamaterials and related devices.
Switchable and tunable metamaterials are expanding areas of research driven by the development of nanophotonic all-optical data processing circuits, optical memory, smart surfaces, adaptable detection, imaging systems, and transformation optics devices [ref.1]. Several avenues are being explored. Metamaterials where metal nanostructures which support plasmons are hybridized with nonlinear and switchable layers provide a way to achieve high-contrast optical switching and enhanced nonlinear responses. Indeed, a change in the refractive index or absorption in a hybridized material will modify the plasmon spectrum of the nanostructure. This can lead to a strong change in the resonant transmission and reflection characteristics of the hybrid structure. For instance the ability to change a metamaterial's response at terahertz frequencies by injection or optical generation of free carriers in a semiconductor substrate has been reported [refs. 2,3]. A layer of single-wall semiconductor carbon nanotubes deposited on a metamaterial shows an order of magnitude higher nonlinearity than the already extremely strong response of the nanotubes themselves due to resonant plasmon-exciton interactions [ref. 4]. Nanoscale metamaterial electro-optical switches using phase change chalcogenide glass [ref. 5] and vanadium dioxide [ref. 6] have already been demonstrated. Graphene is also a popular material that promises to add electrooptical capability to metamaterials in particular in the infrared and terahertz domains by exploiting the spectral shift of the electromagnetic response that is driven by applied voltage [ref. 7,8].
When high-speed control is not needed, metamaterials can be reliably and reversibly controlled by using microelectromechanical (MEMS) actuators to reposition parts of the meta-molecules. MEMS-based metamaterials can provide continuous tuning, rather than steplike switching associated with phase-change materials and, in contrast to approaches exploiting optical nonlinearities, they are compatible with low intensities. This has been convincingly demonstrated for terahertz and far infrared metamaterials consisting of specially designed deformable meta-molecules [refs. 9,10]. It has also been proposed to tune metamaterials structurally through continuous adjustment of a metamaterial's lattice structure [ref. 12]. In ref. 12, this concept is demonstrated in the GHz region by a millimeter scale three-dimensional lattice formed by a vertical stack of circuit boards, each representing a two-dimensional metamaterial, where the tuning is provided by laterally shifting every other circuit board.
However, reconfigurable photonic metamaterials (RPMs) operating in the visible and near-infrared parts of the spectrum require the development of components and actuators operating on the scale of a few tens of nanometers rather than millimeters.